The culprit in this case turned out to be bad u-values for the walls doors of the walk-in coolers in this supermarket model. This resulted in a very large heat removal from the adjacent zones causing a very cold air temperature to be passed to the psychrometric function which tripped the fatal error.

How did we find this?

1. The full error message included "tdb=[-427.76]" which gives a clue that something is very cold.
2. The autosized heating supply air flow rate for some zones was very high (e.g. 1107880 m3/s). This gives a clue that this is an envelope or internal gains problem, because the full system is not modeled during sizing.
3. The refrigerated cases and walkin coolers were the likely cause of the large heating load. Tested this by deleting all refrigeration objects, and the error was gone.
4. After poring over the details of all the refrigerated cases, @apetersen found the problem in the walkin coolers. Every one of them had these inputs:

```

Refrigeration:Walkin,
255.599352755867,        !- Insulated Surface U-Value Facing Zone 1 {W/m2-K}
255.599352755867,        !- Glass Reach In Door U Value Facing Zone 1 {W/m2-K}
255.599352755867,        !- Stocking Door U Value Facing Zone 1 {W/m2-K}

```

Hello Colleagues,

Yes. I found this to be the case to. When I got that error it ended up because of the U value for the Walkin was all off. It seemed to happen when working in OpenStudio at IP units. I found that by switching back to metric units before I simulate the U=value I assigned would stay and not default to the large 255 number that errors out the program.

I used 0.285 W/m2-K = R-20 (U=0.05 Btu/hr-ft2-R) and it ran no problem.

Josh